This description relates to separation and manipulation of a chiral object.
We use the term chiral object (or system) very broadly to include, for example, any object or system that differs from its mirror image such that its mirror image cannot be superimposed on the original object. One kind of chiral object is a chiral molecule, also called an enantiomer. A common feature of chiral molecules is their “handedness” (i.e., right-handed or left-handed). Enantiomers are a subset of chiral objects called stereoisomers. A stereoisomer is one of a set of isomeric molecules whose atoms have similar connectivity but differ in the way the atoms are arranged in space. A stereoisomer includes at least one stereocenter, which is any atom that bears groups such that an interchanging of any two groups leads to a stereoisomer. A stereoisomer may have one or more stereocenters. For example, for a molecule with 3 stereocenters (e.g. S, S, S), its enantiomer would be (R, R, R); epimers are stereoisomers that differ in only one but not all stereocenters (e.g., S, S, R instead of S, S, S). It is also possible for a molecule to be chiral without having a stereocenter (the most common form of chirality in organic compounds). In axial chirality, for example, a molecule does not have a stereocenter but has an axis of chirality, i.e., an axis about which a set of substituents is held in a spatial arrangement that cannot be superimposed on its mirror image. An example is the molecule 1,1′-bi-2-naphthol (BINOL). Although the discussion here refers to enantiomers, it also applies to other stereoisomers, even if they do not qualify as enantiomers. We use the term stereoisomers broadly.
A mixture of molecules is often called racemic if it contains equal amounts of right-handed and left-handed enantiomers, and it is called enantiopure (or, optically pure) if only one type of enantiomer dominates in the mixture. Here, however, we refer to a racemic mixture more broadly to include any non-pure mixture of stereoisomers that is not enantiopure, whether or not the amounts of the stereoisomers are equal.
Chirality is important in chemistry, especially for biological and drug applications. Natural biomolecules are typically found in only one enantiomer form (e.g., proteins, peptides, and amino acids are left-handed, and sugars are right-handed). The fields of drug discovery, development, and manufacturing are interested in molecules that are enantiopure, because one form or enantiomer may work better in vivo while the opposite form may be toxic or may cause side effects. Other chemistry-based fields would also benefit from enantiopure molecules including (for illustration purposes), but not limited to: flavors and fragrances, agrichemicals, fine chemicals, petrochemicals, and others.
Enantiopure samples are sometimes produced by asymmetric synthesis, in which only one form of enantiomer is chemically synthesized from the beginning. Another approach is to synthesize both enantiomers (for example, in a racemic mixture) and then separate the desired enantiomer from the mixture, for example, using column chromatography, in which the mixture is run through a chiral selector (e.g., a chemical matrix that binds preferentially to one enantiomer and less so to its counterpart) iteratively until a desired purity is reached.
Some molecular separation techniques are not effective for chiral molecules, because two counterpart enantiomers generally share physical properties, including chemical composition, charge, size, electric and magnetic dipole moments, and energy levels. Detection and separation of chiral molecules is typically done by interacting the molecules with a chiral medium (e.g., a chemical matrix). Enantiomers also can be identified by their interaction with a chiral (e.g., circularly polarized) electromagnetic field.